The present invention relates to superconductor-metal laminates and to a method of making such laminates.
The high T.sub.c (superconducting transition temperature) superconductors are generally brittle, hard to shape, and high in electrical resistivity at normal temperatures. By high T.sub.c superconductors are meant superconductors with T.sub.c above the temperature of liquid nitrogen (77.degree. K.). Typically, metals are not high T.sub.c superconductors and at normal temperatures are ductile, formable, low in electrical resistivity and high in thermal conductivity.
It has been recognized that the combination of a high T.sub.c superconductor and a metal in the form of a composite material is attractive for the following reasons: the combination has improved toughness, ductility, shapeability and formability; improved electrical and thermal stabilization; and improved critical current density. The critical current density is the current density above which the superconductor loses its superconductivity, at a given temperature below T.sub.c.
Powder metallurgy has been used to fabricate superconductor-metal composites. (In-Gann Chen, S. Sen and D. M. Stefanescu, Appl. Phys. Lett. 52 (16), 1355 (1988); F. H. Streitz, M. Z. Cieplak, Gang Xiao, A. Gavrin, A. Bakhshai and C. L. Chien, Appl. Phys. Lett. 52, 927 (1988); A. Goyal, P. D. Funkenbusch, G. C. S. Chang and S. J. Burns, Mater. Lett. 6 (8-9), 257 (1988)). Generally powder metallurgy involves mixing superconductor powder and metal powder, followed by sintering the mixture. There are some problems with this method. For example, the metal content is limited to 50 vol. % or below in order to have a continuous superconducting path in the composite. This limits the ductility of the composite. In addition, the choice of metal is limited to metals that are stable at the sintering temperature in oxygen and do not react with the superconductor at the sintering temperature (typically 950.degree. C. for YBa.sub.2 Cu.sub.3 O.sub.7-.delta.).
Another method that has been used in forming a superconductor-metal composite involves (i) packing a superconductor powder in a metal tube, (ii) drawing the tube to a smaller diameter, and (iii) sintering. This method is commonly known as the metal tube drawing method. (R. W. McCallum, J. D. Verhoeven, M. A. Noack, E. D. Gibson, F. C. Laabs and D. K. Finnemore, Advanced Ceramic Materials 2 (3B), 388 (1987); S. Jin, R. C. Sherwood, R. B. Van Dover, T. H. Tiefel and D. W. Johnson, Jr., Appl. Phys. Lett. 51, 203 (1987)). Similar drawbacks as with the powder metallurgy method have been recognized with this method. For example, the choice of metal is limited to metals that are stable at the high sintering temperature required by the superconductor.
A further disadvantage of the superconductor-metal composites formed by the above described methods is that they are generally weak in tension.
It is therefore an object of this invention to provide a method for fabricating superconductor-metal materials which overcome the problems recognized in the art.
It is a further object to provide a method of producing a superconductor-metal material which overcomes the limitations of known methods while retaining the desirable characteristics.
A still further object is to produce a superconductor-metal material having both good tensile and compressive strength.